Photoelectric multiplier tube of reduced length

ABSTRACT

This invention relates to a photomultiplier tube including: a photocathode PK with a semi-transparent photo-sensitive layer provided to emit an electron flux towards the inside of the tube, focusing optics comprising a first dynode D 1 , concave on the side of the photocathode PK, and several Rajkman dynodes D 3 , . . . , D 8  located on each side of a plane called the dynodes plane DP. According to the invention, the focusing optics also includes a second dynode D 2  concave on the side of the re-emitting surface of the first dynode D 1,  the angle between the plane of the dynodes DP and the center line of the tube exceeding 45°, the concave side of the first Rajkman dynode D 3  facing the re-emitting surface of the second dynode D 2.

This invention relates to a photomultiplier tube comprising:

a photocathode designed to be raised to a first electrical potential andwith a semi-transparent photo-sensitive layer designed to receive anillumination from outside the tube and to transmit an electron flux intothe tube, the density of the flux depending on the intensity of theillumination received by the photocathode,

focusing optics comprising a first dynode that will be raised to asecond electrical potential, the value of which is higher than the firstpotential, that is provided with a “re-emitting” surface composed of amaterial encouraging secondary emission phenomena, the said surfacebeing concave on the side of the photocathode, and,

several Rajkman dynode laid out on each side of a plane called thedynode plane. The first of the dynode closest to the output from thefocusing optics will be raised to a third electrical potential, thevalue of which is higher than the potential of the second dynode, eachof the subsequent dynode will be raised to an electrical potentialhigher than the potential of the preceding dynode, this series of dynodebeing designed to receive and amplify the electron flux from thefocusing optics.

In most photomultiplier tubes that use Rajkman dynode based on theprinciple described above, the dynode plane is parallel to the centerline of the tube. Therefore the dimension of the tube along this axis,called the tube length, is large. This may be prohibitive for manyapplications, for example when the tube is used within a gamma-camerafor detection of radiation, it is desirable to use short tubes in orderto reduce the size of the device in which they are fitted.

The purpose of the invention is to overcome this disadvantage byproposing a photomultiplier tube in which the plane of the dynode is notparallel to the center line of the tube.

A photomultiplier tube as described in the introductory paragraph ischaracterized according to this invention in that the focusing opticsalso comprise a second dynode that will be raised to a potential whichis intermediate between the potential of the second and third dynode,the second dynode having a concave re-emitting surface on the side ofthe re-emitting surface of the first dynode, and in that the anglebetween the plane of the dynode and the center line of the tube, definedas being a center line perpendicular to the photocathode at itsmidpoint, exceeds 45°, the concave side of the first Rajkman dynodefacing the re-emitting surface of the second dynode.

In this type of photomultiplier tube, the dimension along the length dueto the series of Rajkman dynode reduces as the angle between the planeof the dynodes and the center line of the tube increases. The seconddynode redirects the electron flux output from the first dynode towardsthe first Rajkman dynode. The second dynode may beneficially be equippedwith a conducting grid placed across the path followed by the electronflux between the first and the second dynode, the potential of this gridbeing made similar to the potential of the second dynode.

In one particular embodiment of the invention, the angle between theplane of the dynodes and the center line of the tube is close to 90°.

With this configuration, the influence of the series of Rajkman dynodesalong the total length of the tube can be reduced by a maximum amount.

In one preferred embodiment of the invention, a photomultiplier tubelike the tube described above is characterized in that it comprises agrid placed between the second dynode and the first Rajkman dynode, thatwill be raised to electrical potential similar to the potential of thesecond Rajkman dynode.

The presence of the grid increases the collection efficiency at thefirst Rajkman dynode, in other words the ratio between the number ofelectrons received by the said dynode and the number of electronstransmitted by the second dynode. The grid generates a local electricfield approximately parallel to the path between the second dynode andthe first Rajkman dynode, which accelerates electrons in itsneighborhood and directs them towards the first Rajkman dynode.

BRIEF DESCRIPTION OF DRAWINGS

The sole FIGURE shows a structure of the Photomultiplier tube accordingto the invention.

The invention will be better understood by means of the followingdescription of one embodiment given as a non-restrictive example withreference to FIG. 1, which diagrammatically shows a sectional view of aphotomultiplier tube according to the invention. The plane of thesection is parallel to an axis TAX, called the tube axis, and isperpendicular to a plane called the dynodes plane, which intersects withthe plane of the section along a line shown on the diagram as DP. Thephotomultiplier tube comprises an external glass casing TU, for examplewhich may have a symmetry of revolution about the center line of tubeTU, and which has a surface perpendicular to the center line of the tubeTAX on which a photocathode PK is fitted that will be raised to a firstelectrical potential and on which a semi-transparent photo-sensitivelayer is formed. This photomultiplier tube also comprises focusingoptics comprising a first dynode D1 that will be raised to a secondelectrical potential at a value that is higher than the first potential,with a “re-emitting” surface composed of a material encouragingsecondary emission phenomena, the said surface being concave on the sideof the photocathode PK. The focusing optics also comprise the seconddynode D2 that will be raised to a potential, the value of which exceedsthe value of the second potential, and which has a concave re-emittingsurface on the side of the re-emitting surface of the first dynode D1.The photomultiplier tube also comprises several Rajkman dynodes D3, . .. , D8, that will receive and amplify the electron flux from thefocusing optics, and dynodes on each side of the plane of the dynodes,the first of which, D3, is closest to the second dynode D2 and whichwill be raised to a third electrical potential, the value of whichexceeds the value of the potential of the second dynode D2. Theconcaveness of the first Rajkman dynode D3 faces the re-emitting surfaceof the second dynode D2. Each of the subsequent dynodes D4, . . . , D8will be raised to an electrical potential that exceeds the potential ofthe preceding dynode. The angle, β, between the center line DP and thecenter line of the tube TAX is close to 90°. Finally, thephotomultiplier tube comprises a grid Gd, for example made of conductingrods, located between the second dynode D2 and the first Rajkman dynodeD3, and which will be raised at an electrical potential similar to thepotential of the second Rajkman dynode D4.

When the photocathode PK is illuminated, and the energy of the receivedphotons is sufficiently high, the photo-sensitive layer emits anelectron flux towards the inside of the tube, the density of which thusdepends on the illumination intensity. These electrons are collected bythe first dynode D1, due to the difference in potential between thefirst dynode D1 and the photocathode PK that creates an electrical fieldfrom the first dynode D1 towards the photocathode PK. The first dynodeD1 re-emits a larger number of electrons than it collects, due tosecondary emission phenomena well known to a specialist in the subject,and thus performs a first amplification of the density of the electronflux. Electrons re-emitted by the first dynode D1 are collected by thesecond dynode D2, due to the difference in potential between the seconddynode D2 and the first dynode D1 which creates an electrical fielddirected from the second dynode D2 towards the first dynode D1.Electrons re-emitted by the second dynode D2 are accelerated by theelectrical field existing locally around the grid Gd, which directs themto the first Rajkman dynode D3, which thus has a very high collectionefficiency. Finally, the electron flux is subject to successiveamplifications made by Rajkman dynodes according to a process known toan expert in the subject, and which there is no need to develop here,before reaching an anode AN that forms the output from the tube andrestores electronic information representing the illumination receivedby the photocathode PK.

Therefore, the structure of the focusing optics D1, D2, is such that theelectron flux can be redirected towards the first Rajkman dynode whenthe angle between the plane of the dynodes and the center line of thetube TAX is large. The usefulness of this arrangement is obvious in thisexample, in which the angle β is close to 90°, so that the lengthnecessary for the series of Rajkman dynodes D3, . . . , D8 can beminimized, thus minimizing the total length of the tube.

What is claimed is:
 1. Photomultiplier tube comprising: a photocathode(PK) designed to be raised to a first electrical potential and providedwith a semi-transparent photo-sensitive layer designed to receive anillumination from outside the tube and to transmit an electron flux intothe tube, the density of the flux depending on the intensity of theillumination received by the photocathode, focusing optics comprising afirst dynode (D1) configured to be raised to a second electricalpotential, the value of which is higher than the first potential, thatis provided with a “re-emitting” surface composed of a materialencouraging secondary emission phenomena, the said surface being concaveon the side of the photocathode, and, several Rajkman dynodes (D3 to D8)laid out on each side of a plane (DP) called the dynodes plane, thefirst of the said several Rajkman dynodes (D3) being closest to theoutput from the focusing optics raised to a third electrical potential,the value of which is higher than the second potential, each of thesubsequent dynodes raised to an electrical potential higher than thepotential of the preceding dynode, this series of dynodes being designedto receive and amplify the electron flux from the focusing optics, thefocusing optics also comprising a second dynode (D2) configured to beraised to a potential which is intermediate between the second and thirdpotentials, he second dynode having a concave re-emitting surface on theside of the re-emitting surface of the first dynode (D1), the anglebetween the plane of the dynodes (DP) and the tube axis (TAX), definedas being an axis perpendicular to the photocathode at its midpoint,exceeding 45°, characterized in that the concave side of the firstRajkman dynode (D3) faces the re-emitting surface of the second dynode(D2).
 2. Photomultiplier tube according to claim 1, characterized inthat the angle between the plane of the dynodes (DP) and the tube axis(TAX) is close to 90°.
 3. Photomultiplier tube according to one ofclaims 1 or 2, characterized in that it comprises a grid (Gd) locatedbetween the second dynode (D2) and the first Rajkman dynode (D3) andconfigured to be raised to an electrical potential similar to theelectrical potential of the second Rajkman dynode (D4). 4.Photomultiplier tube according to claim 3, in which the grid (Gd) iscomposed of conducting bars.